The ternary complex of integrin-linked kinase (ILK), PINCH and parvin functions as a signalling platform for integrins by interfacing with the actin cytoskeleton and many diverse signalling pathways. All these proteins have synergistic functions at focal adhesions, but recent work has indicated that these proteins might also have separate roles within a cell. They function as regulators of gene transcription or cell-cell adhesion.

The growing number of recently identified negative feedback regulators of receptor tyrosine kinases (RTKs) highlights the importance of signal attenuation and modulation for correct signaling outcome. Mitogen-inducible gene 6 (Mig6 also known as RALT or Gene 33) is a multiadaptor protein thought to be involved in the regulation of RTK and stress signaling. Here, we show that deletion of the mouse gene encoding Mig6 (designated Errfi1, which stands for ERBB receptor feedback inhibitor 1) causes hyperactivation of endogenous epidermal growth factor receptor (EGFR) and sustained signaling through the mitogen-activated protein kinase (MAPK) pathway, resulting in overproliferation and impaired differentiation of epidermal keratinocytes. Furthermore, Errfi1-/- mice develop spontaneous tumors in various organs and are highly susceptible to chemically induced formation of skin tumors. A tumor-suppressive role for Mig6 is supported by our finding that MIG6 is downregulated in various human cancers. Inhibition of endogenous Egfr signaling with the Egfr inhibitor gefitinib (Iressa) or replacement of wild-type Egfr with the kinase-deficient protein encoded by the hypomorphic Egfr(wa2) allele completely rescued skin defects in Erffi1-/- mice. Carcinogen-induced tumors displayed by Errfi1-/- mice were highly sensitive to gefitinib. These results indicate that Mig6 is a specific negative regulator of Egfr signaling in skin morphogenesis and is a novel tumor suppressor of Egfr-dependent carcinogenesis.

Recent biochemical studies indicate that the serotonin transporter can form oligomers. We investigated whether the human serotonin transporter (hSERT) can be visualized as an oligomer in the plasma membrane of intact cells. For this purpose, we generated fusion proteins of hSERT and spectral variants of the green fluorescent protein (cyan and yellow fluorescent proteins, CFP and YFP, respectively). When expressed in human embryonic kidney 293 cells, the resulting fusion proteins (CFP-hSERT and YFP-hSERT) were efficiently inserted into the plasma membrane and were functionally indistinguishable from wild-type hSERT. Oligomers were visualized by fluorescence resonance energy transfer microscopy in living cells using two complementary methods, i.e. ratio imaging and donor photobleaching. Interestingly, oligomerization was not confined to hSERT; fluorescence resonance energy transfer was also observed between CFP-and YFP-labeled rat ␥-aminobutyric acid transporter. The bulk of serotonin transporters was recovered as high molecular weight complexes upon gel filtration in detergent solution. In contrast, the monomers of CFP-hSERT and YFP-hSERT were essentially undetectable. This indicates that the homo-oligomeric form is the favored state of hSERT in living cells, which is not significantly affected by coincubation with transporter substrates or blockers. Based on our observations, we conclude that constitutive oligomer formation might be a general property of Na ؉ / Cl ؊ -dependent neurotransmitter transporters.It is widely accepted that tyrosine kinases and related receptors signal as dimers (1). Similarly, the oligomeric nature of voltage-dependent and ligand-gated ion channels is firmly established. In addition, over recent years it has been determined that other integral membrane proteins, which were originally thought to exist in monomeric form, actually form homo-and hetero-oligomers. For instance, this is true for several G protein-coupled receptors (2-5) and for a number of transporters, e.g. the erythrocyte glucose transporter-1 and the brain glutamate transporter (6, 7). The structural organization of transporters is likely to determine their function. This consideration is particularly relevant in understanding the transporters that mediate the re-uptake of neurotransmitters from the synaptic cleft (8). These proteins depend on the presence of Na ϩ and Cl Ϫ and generate a current during transport, i.e. they may share properties similar to ion channels (9 -12) that are known to be organized as oligomeric complexes. The human serotonin transporter (hSERT) 1 is a prototypic member of this family; its properties are of considerable clinical interest because the inhibitors are useful as antidepressants, and substrates that induce the reversal of transport (e.g. "ecstasy") are abused (13). The complexity of the transport reaction is suggestive of a higher level of organization, and recent biochemical experiments on the SERT of different species indicate that the transporter can, in principle, form oligomeric str...

The serotonin transporter (SERT) terminates neurotransmission by removing serotonin from the synaptic cleft. In addition, it is the site of action of antidepressants (which block the transporter) and of amphetamines (which induce substrate efflux). We explored the functional importance of the N terminus in mediating the action of amphetamines by focusing initially on the highly conserved threonine residue at position 81, a candidate site for phosphorylation by protein kinase C. Molecular dynamics simulations of the wild type SERT, compared with its mutations SERTT81A and SERTT81D, suggested structural changes in the inner vestibule indicative of an opening of the inner vestibule. Predictions from this model (e.g. the preferential accumulation of SERTT81A in the inward conformation, its reduced turnover number, and a larger distance between its N and C termini) were verified. Most importantly, SERTT81A (and the homologous mutations in noradrenaline and dopamine) failed to support amphetamine-induced efflux, and this was not remedied by aspartate at this position. Amphetamine-induced currents through SERTT81A were comparable with those through the wild type transporter. Both abundant Na+ entry and accumulation of SERTT81A in the inward facing conformation ought to favor amphetamine-induced efflux. Thus, we surmised that the N terminus must play a direct role in driving the transporter into a state that supports amphetamine-induced efflux. This hypothesis was verified by truncating the first 64 amino acids and by tethering the N terminus to an additional transmembrane helix. Either modification abolished amphetamine-induced efflux. We therefore conclude that the N terminus of monoamine transporters acts as a lever that sustains reverse transport.

The serotonin transporter (SERT) is a member of the SLC6 family of solute carriers. SERT plays a crucial role in synaptic neurotransmission by retrieving released serotonin. The intracellular carboxyl terminus of various neurotransmitter transporters has been shown to be important for the correct delivery of SLC6 family members to the cell surface. Here we studied the importance of the C terminus in trafficking and folding of human SERT. Serial truncations followed by mutagenesis identified sequence spots (PG601,602, RII607–609) within the C terminus relevant for export of SERT from the endoplasmic reticulum (ER). RI607,608 is homologous to the RL-motif that in other SLC6 family members provides a docking site for the COPII component Sec24D. The primary defect resulting from mutation at PG601,602 and RI607,608 was impaired folding, because mutated transporters failed to bind the inhibitor [3H]imipramine. In contrast, when retained in the ER (e.g. by dominant negative Sar1) the wild type transporter bound [3H]imipramine with an affinity comparable to that of the surface-expressed transporter. SERT-RI607,608AA and SERT-RII607–609AAA were partially rescued by treatment of cells with the nonspecific chemical chaperone DMSO or the specific pharmacochaperone ibogaine (which binds to the inward facing conformation of SERT) but not by other classes of ligands (inhibitors, substrates, amphetamines). These observations (i) demonstrate an hitherto unappreciated role of the C terminus in the folding of SERT, (ii) indicates that the folding trajectory proceeds via an inward facing intermediate, and (iii) suggest a model where the RI-motif plays a crucial role in preventing premature Sec24-recruitment and export of incorrectly folded transporters.

D 1 and D 2 receptors, the "classical" dopamine receptor subtypes, are abundantly expressed in the basal ganglia and are important targets in pharmacotherapy, yet the basis for their neuromodulatory effects is not well understood at the cellular level. The D 2 -dopamine receptor is found (as an autoreceptor) on presynaptic nerve terminals of nigrostriatal projections and, postsynaptically, on the medium spiny neuron, the predominant nerve cell of the neostriatum (2). The excitatory drive for the medium spiny neuron is provided by glutamatergic afferents which through NMDA receptors trigger Ca 2ϩ influx (3). Hence, neuronal signal transduction by dopamine receptors proceeds in the presence of oscillating intracellular Ca 2ϩ concentrations and there is reason to assume that the signaling mechanism is interrelated with the intracellular Ca 2ϩ level. Calmodulin (CaM), 1 a small acidic protein, can be considered the primary decoder of Ca 2ϩ information in the cell. CaM has a Ca 2ϩ affinity of 10 Ϫ6 M and thus acts as a switch when the concentration rises from a resting value of ϳ10 Ϫ7 M to 10Calmodulin can be activated by persistent elevation of intracellular Ca 2ϩ and by Ca 2ϩ oscillations, as they occur on repeated depolarizations of nerve cells (4). It has long been known that major effectors regulated by the D 2 -dopamine receptor can be regulated by Ca 2ϩ and that these effector molecules are enriched in striatal neurons. In these instances, increases in Ca 2ϩ levels elicit effects similar to D 2 receptor activation. For example, Ca 2ϩ reduces the intracellular cAMP levels by inhibiting adenylyl cyclase type V (and type VI) and by activating CaM-sensitive phosphodiesterases, which break down cAMP; both type V adenylyl cyclase (5, 6) and a 63-kDa isoform of phosphodiesterase (PDE1B1) are expressed in striatal neurons (7,8). Another example for the cross-talk between D 2 receptor signaling and Ca 2ϩ /CaM is the target protein DARPP-32, an inhibitor of protein phosphatase 1. DARPP-32 is dephosphorylated on D 2 -dopamine receptor activation and thus becomes active; this effect is strongly enhanced by Ca 2ϩ /CaM through activation of calcineurin (9). These examples suggest that the signal transduced by Ca 2ϩ /CaM and signaling initiated by the intracellular D 2 receptor overlap and may add to each other.We have found in the primary peptide sequence of the human D 2 -dopamine receptor a CaM-binding motif, which is located in the NH 2 terminus of the third cytoplasmic loop of the receptor. In the present work, we report that CaM can convey * This work was supported by Austrian Science Foundation Grants P13097 (to M. F.) and P14273 (to C. N.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.¶ To whom correspondence should be addressed. E-mail: christian. nanoff@univie.ac.at.

The transporters for serotonin (SERT), dopamine, and noradrenaline have a conserved hydrophobic core but divergent N and C termini. The C terminus harbors the binding site for the coat protein complex II (COPII) cargo-binding protein SEC24. Here we explored which SEC24 isoform was required for export of SERT from the endoplasmic reticulum (ER). Three lines of evidence argue that SERT can only exit the ER by recruiting SEC24C: (i) Mass spectrometry showed that a peptide corresponding to the C terminus of SERT recruited SEC24C-containing COPII complexes from mouse brain lysates. (ii) Depletion of individual SEC24 isoforms by siRNAs revealed that SERT was trapped in the ER only if SEC24C was down-regulated, in both, cells that expressed SERT endogenously or after transfection. The combination of all siRNAs was not more effective than that directed against SEC24C. A SERT mutant in which the SEC24C-binding motif (607RI608) was replaced by alanine was insensitive to down-regulation of SEC24C levels. (iii) Overexpression of a SEC24C variant with a mutation in the candidate cargo-binding motif (SEC24C-D796V/D797N) but not of the corresponding mutant SEC24D-D733V/D734N reduced SERT surface levels. In contrast, noradrenaline and dopamine transporters and the more distantly related GABA transporter 1 relied on SEC24D for ER export. These observations demonstrate that closely related transporters are exclusive client cargo proteins for different SEC24 isoforms. The short promoter polymorphism results in reduced SERT cell surface levels and renders affected individuals more susceptible to depression. By inference, variations in the Sec24C gene may also affect SERT cell surface levels and thus be linked to mood disorders.

Nerve functions require phosphatidylinositol-4,5-bisphosphate (PIP 2 ) that binds to ion channels, thereby controlling their gating. Channel properties are also attributed to serotonin transporters (SERTs); however, SERT regulation by PIP 2 has not been reported. SERTs control neurotransmission by removing serotonin from the extracellular space. An increase in extracellular serotonin results from transporter-mediated efflux triggered by amphetamine-like psychostimulants. Herein, we altered the abundance of PIP 2 by activating phospholipase-C (PLC), using a scavenging peptide, and inhibiting PIP 2 -synthesis. We tested the effects of the verified scarcity of PIP 2 on amphetamine-triggered SERT functions in human cells. We observed an interaction between SERT and PIP 2 in pull-down assays. On decreased PIP 2 availability, amphetamine-evoked currents were markedly reduced compared with controls, as was amphetamineinduced efflux. Signaling downstream of PLC was excluded as a cause for these effects. A reduction of substrate efflux due to PLC activation was also found with recombinant noradrenaline transporters and in rat hippocampal slices. Transmitter uptake was not affected by PIP 2 reduction. Moreover, SERT was revealed to have a positively charged binding site for PIP 2 . Mutation of the latter resulted in a loss of amphetamine-induced SERT-mediated efflux and currents, as well as a lack of PIP 2 -dependent effects. Substrate uptake and surface expression were comparable between mutant and WT SERTs. These findings demonstrate that PIP 2 binding to monoamine transporters is a prerequisite for amphetamine actions without being a requirement for neurotransmitter uptake. These results open the way to target amphetamine-induced SERT-dependent actions independently of normal SERT function and thus to treat psychostimulant addiction.phosphoinositide | reuptake | release | mass spectrometry | amperometry